On Jun 29, 2009, at 8:30 AM, Adam wrote:
> > This is a lovely 700Mhz G3 (the last G3 made) that has no trouble > running 10.3 with 500MB of RAM. Normally. <snip> > > With increasing frequency, the computer shuts down without warning. > The video goes, there is an audible "thunk," and the hard drive goes > silent. It's definitely not just the video that's out. > > iMac won't reboot right away, but will eventually after shutdown. But > often (though not always), after a time that typically ranges from 10 > - 60 minutes, the video returns and the hard drive starts to spin > again. It's completely frozen, but will reboot at that point. > > Once rebooted it often will run without incident for days. But, the > behavior is variable. Sometimes it won't boot for days. > > Using the CD drive often seems to bring on this shutdown--but not > always. > > I think I can rule out a software problem. I have repaired the disk, > the permissions, run Disk Warrior, and reinstalled OS X. I have zapped > the parameter RAM. The firmware is up-to-date, as far as I can tell > (also this is a recent prblem). I have reset the NVRAM in open > firmware and the PMU on the mother board. The battery seems fine. I've > played will all sorts of variation on the power-saver preferences, > about going into sleep (or not) and turning off the hard drive (or > not). > > I've removed and reinstalled the two RAM chips and dismantled down to > the hard drive (and reassembled), checking for anything loose. I > didn't want to go further without some idea of what might need to be > replaced. Something in the video side? The power supply? <snip> I've only trimmed your post a bit in order to give context to my comments. Intermittents are always a bear to diagnose, but from your description it's clear that your iMac loses power abruptly, and that the shutdown follows a period of "normal" operation. This points to a short (or shorts) caused by heat, which is an assumption reinforced by the fact your iMac will restart after a cooling off period that ranges from 10 to 60 minutes, but sometimes for days. Another hint is that using the CD drive *sometimes* brings on the abrupt shutdown. Unless you're also seeing color shifts and video artifacts (flashes, color bars, lines, etc.), the video side appears to be OK. I'd also give everything else *except* your PAV board an OK. PAV stands for Power Analog Video, and is actually two "boards," not one, which gets confusing. The video board is affixed to the neck of the cathode ray tube (CRT), but is connected directly to the power and analog board via cables. The fact you see stable video when the iMac's running pretty much eliminates the video board as a culprit. The fact you've been through the software and checked everything else eliminates all those items. The only thing left (with one caveat which I'll get to later) is the Power and Analog board part of the PAV. This board resides on the top side of the perforated aluminum divider board, and right underneath the CRT neck. The usual culprit on this board is the flyback transformer (FBT), which powers the CRT as well as the video board. After a lot of running cycles, the thin insulation on the wires wound tightly inside the FBT breaks down, which causes intermittent shorts, which causes overheating, which eventually results in a complete no- start or shutdown condition. Normally, a failing FBT will give plenty of warning. It usually starts with snapping noises accompanied by screen flashes, escalating as the deterioration worsens. Acrid burning odors are not unusual, and occasionally there's a puff of smoke as an FBT dies. I've dealt with a number of FBT failures in all-in-one CRT-equipped Macs, specifically G3 AIOs and both early and late G3 iMacs. In most cases, I've been able to restore the dead or failing Macs to good health by replacing the FBT. But I had the advantage of seeing, hearing and usually smelling the failure patterns of the machines as they were still running. However, I've been unable to restore a couple of iMacs with a simple (relatively, that is) FBT replacement. Other components on the PA board had been damaged, but my ability to do component-level repair is non-existent. Like you, I'm only a hobbyist. So I repaired those iMacs with a replacement of the PAV boards with known-good used ones. The video board, which is part of the PAV boards sold on eBay and elsewhere, usually isn't bad but I replace it anyway in order to keep a working pair of boards intact. I've only found one bad video board in dozens and dozens of iMacs with video problems, by the way. So if you find a good used PAV set and replace yours with it, that should restore your iMac to stable running condition. Now here's the caveat I mentioned earlier: A few times, I've found that a PAV replacement cures some of the symptoms, but not all. Once, a PAV replacement didn't fix anything. The rest of the problem was the logic board, which is connected to the PAV board(s). Just yesterday, for example, I repaired a G3/400 iMac which had a no-start condition. Plugged it in, and nothing happened when I pushed the power button. Upon removing the bottom case, I noticed a burned area on the perforated EFI shield above the PRAM battery and video chip. Yep, the video chip and add-on accelerator card had gotten hot enough to melt and burn. So the logic board was literally toast. But when I replaced the logic board, the iMac still refused to start. Dead, dead, dead. Since I knew the hard drive, CDRW, PRAM battery and everything but the PAV was OK, I replaced the PAV and, voila!, the iMac sprang to life. By the way, there were two PAV boards for slot-loading G3 iMacs. Make sure you get one that works with your CRT. HTH, Jim Scott --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed Low End Mac's G3-5 List, a group for those using G3, G4, and G5 desktop Macs - with a particular focus on Power Macs. The list FAQ is at http://lowendmac.com/lists/g-list.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/g3-5-list?hl=en Low End Mac RSS feed at feed://lowendmac.com/feed.xml -~----------~----~----~----~------~----~------~--~---
